The long-term objectives of this proposal are to understand the molecular structure and epidemiology of human parainfluenza virus type one (HPIV-1) as they relate to the host immune response, vaccine development, disease prediction and prevention. This virus belongs to the paramyxovirus family, which includes other important human pathogens such as measles, mumps, respiratory syncytial virus, and parainfluenza virus type 3. HPIV-1 is the most common cause of croup in infants and young children generating epidemics throughout the world on a biennial basis. Specific aims of the proposed research are: 1; To determine the genetic and antigenic diversity in the HN (hemagglutinin-neuraminidase) of HPIV-1 clinical isolates collected over 25 years; 2; To determine the genetic and antigenic diversity in a single HPIV-1 population associated with epidemic croup; 3; To determine in children the distribution of serum antibody directed toward the previously mapped HPIV-1 antigenic sites before and after a croup epidemic. The effort in this proposal will be directed toward characterizing antigenically and genetically the HN molecule of HPIV-1. This surface glycoprotein probably plays an important role in HPIV-1 pathogenesis, as do similar molecules in other closely related viral diseases. It is the target for a large portion of the protective antibody response in humans, and could contain important epidemiologic markers. The characterization of this protein will involve examining viral populations over time, from different geographic locations, and during an epidemic. Fourteen anti-HN monoclonal antibodies will be used to examine these HPIV-1 isolates and to compare their HN proteins to the antigenic map of the 1957 type strain. Similarly, the RNA sequence of this type strain will be used to judge genetic changes. Possible strains of HPIV-1 will be identified as will any differences in virulence. Genetically and antigenically stable neutralizing antigenic sites will be sought along with confirmation of their immunodominance in children. Together, this information will provide a reliable foundation of molecular epidemiology and pathogenesis that should yield the first clues to structure/function relationships among the HN gene, HN protein and neutralizing antigenic sites, and point the way toward modern vaccine strategies. This proposal will provide important information in 3 of the 4 areas of research recently recommended for HPIV-1 by a combined NIAID and WHO workshop.